With the advancement in portable electronic devices and intense interest in plug-in hybrid electric vehicles, there is great demand to increase the energy and power capabilities of lithium ion batteries. In this regard, the 5 V spinel cathode LiMn2−xMxO4 (where M is e.g. Co, Cr, Ni, Fe, Cu or Ga, and x is about 0.5) has drawn much attention due to its high operating voltage and the high intrinsic rate capability offered by the 3-dimensional lithium ion diffusion in the spinel lattice. Moreover, the difficulties encountered with the dissolution of manganese and Jahn-Teller distortion in the 4 V LiMn2O4 cathode are suppressed in LiMn2−xMxO4 as it contains less Mn3+ in the material. In this regard, a 5 V spinel cathode such as LiMn1.5Ni0.5O4 is very attractive due to a nearly flat operating voltage close to 5 V and an acceptably high capacity arising from operation of the Ni2+/3+ and Ni3+/4+ redox couples.
Even a LiMn2−xMxO4 cathode active material suffers from stability problems, however, including the structural instability problems sometimes seen in cation ordered LiMn1.5Ni0.5O4 material, and the surface instability problems sometimes caused by the reaction with electrolyte. Problems such as these can significantly degrade the electrochemical performance.
Partial substitution of Mn and/or Ni in LiMn1.5Ni0.5O4 by other elements such as Li, Al, Mg, Ti, Cr, Fe, Co, Cu, Zn or Mo has been pursued to improve the cyclability. Some of these substitutions improve the cyclability due to the stabilization of the spinel lattice with a disordering of the cations in the 16d octahedral sites, and a smaller lattice parameter difference among the three cubic phases formed during cycling. Although the structural stability of LiMn1.5Ni0.5O4 can be improved by proper cation partial substitution, chemical instability still remains as a problem.
A need thus remains for improved performance in a balance of several different properties as exhibited by the LiMn1.5M0.5O4 spinel cathode material.